1. Field of the Invention
The present invention relates to measuring electrical impedance, and particularly electrode impedance used to acquire physiological signals. The measurement of electrode impedance is typically performed to ensure proper electrode-to-skin contact, and thus verify the quality of the acquired signals. It can also be used to determine skin conductivity, which is a function of physiological processes such as sweating, and can be used to determine the level of autonomic activation as a result of psychological or physiological stress.
The present invention relates in particular to a continuous method for performing such measurement. The measurement is performed in such a way that it does not affect the bioband of the signal. In this application, we define the bioband as the range (or ranges) of frequencies that contains components used for diagnostic, prognostic, triage, and/or treatment purposes. The method and system disclosed herein further provides the means to perform the measure of the impedance of each single electrode. Finally, this measurement can be done either at one particular frequency (including 0 Hz), or at different frequencies simultaneously.
2. Technology Review
Virtually all forms of biomedical/physiological signal monitoring rely on physiological sensors to provide as clear and accurate of a signal as possible. The most commonly used form of these sensors is electrodes, particularly physiological electrodes. Common examples of such physiological signal monitoring techniques include electroencephalography (EEG), electrocardiography (EKG), electromyography (EMG), and electrooculography (EOG), although this list is not exhaustive. Each of these signal monitoring techniques utilizes the placement of physiological electrodes to conduct the corresponding signals to the signal processing hardware. However, the quality of physiological signals, and therefore the accuracy and usefulness of the subsequent signal processing, is subject to the quality of the connection between the electrode and the subject or patient's body, the quality of the connection between the monitoring device and electrode as well as the fidelity of the electrode itself.
Poor physiological signal quality can result in high impedance which in turn decreases the quality or strength of the signal received. High impedance detrimentally affects physiological signal quality by potentially making it more difficult to distinguish a weaker physiological signal from artifacts or other noise in the system. If impedance is particularly high, the signal may not even be conducted through the electrode and therefore may not reach the monitoring equipment at all.
Additionally, artifacts may be created if signal quality decreases and impedance rises. One example is if an electrode moves from its position on the subject or patient's skin and therefore the signal is transmitted intermittently. Not only is impedance increased, perhaps on an intermittent basis, as a result of the break in connection between the electrode and the subject or patient's skin, but the movement of the electrode may create artifacts in the signal when it is properly recorded. Although there are signal processing techniques to remove or correct artifacts, it is of course better to minimize the occurrence of such artifacts.
These issues can potentially lead to inaccurate analysis, which in turn can create potentially serious problems such as incorrect or missed diagnoses of illness, or other physiological manifestations. Ensuring fidelity of signal quality starting with the electrodes conducting these physiological signals is an important issue. Measuring impedance to maintain good signal quality, however, generally interrupts the signal monitoring process. Supplying an electrical current to an electrode to measure impedance typically causes an artifact, spike or perturbation in the monitored physiological signal. Not only does that perturbation interfere with the signal itself, but it causes the data at various points of time surrounding it to become corrupted. Usually, the perturbation and the portions of the signal affected by it must be removed from the signal and therefore gaps are created in the data being collected. When there are gaps in the data there may be significant or important physiological activity that may be missed lead to incorrect or missed diagnoses, improper treatment, and the like.
Perturbation is generally caused by supplying or removing a current for measuring impedance to the electrodes substantially instantaneously; that is, the current is supplied, in effect, at full load. In other words, the current resembles a square wave where the current increases to full load when it is turned on and to no load when it is turned off. When supplying the current this way to check for impedance problems, the electrical pulse disrupts, or perturbs, the electrical EEG signal being monitored, and as it is proportionally much larger or stronger than the physiological EEG signal, and that physiological signal is lost within the artifact created by the electrical pulse. A period of time is necessary for the signal to recover from the perturbation as the oscillations lose power over time. This time period is the gap where data is lost.
It is therefore an object of the present invention to provide a system and method that meets all of these needs and others where such a device and method would be applicable. It is another object of the present invention that this system and method be capable of checking electrical impedance in physiological electrodes both manually on demand as well as in an automated fashion, and both in a substantially continuous manner. It is also an object of the present invention that that this system and method minimize data loss from impedance measurement. Finally, it is an object of the present invention that physiological signals are transmitted correctly and efficiently to increase the accuracy and fidelity of the signal processing methods and ensure accurate diagnosis and treatment.